For configuring a demultiplexing apparatus for demultiplexing a received signal including a plurality of frequency-multiplexed signals to output demultiplexed signals or configuring a multiplexing apparatus for frequency-multiplexing a plurality of input signals to output a multiplexed signal by using analog circuits, the same number of filter and frequency conversion circuits as the number of signals become necessary. Thus, there is a problem in that apparatus size and adjusting parts of the apparatus increase. As a method for solving this problem, a digital signal demultiplexing apparatus and a digital signal multiplexing apparatus which are collective type and use digital signal processing circuits are proposed (refer to patent document 1, for example).
FIG. 1 is a block diagram of the digital signal demultiplexing apparatus described in the patent document 1, and shows a case for demultiplexing up to eight signals to output them. According to FIG. 1, the digital signal demultiplexing apparatus hierarchically connects 2-demultiplexing filter banks 81-87 each having one input and two outputs. In more detail, the second stage 2-demultiplexing filter banks 82 and 83 are connected to the first stage 2-demultiplexing filter bank 81, the third stage 2-demultiplexing filter banks 84 and 85 are connected to the second stage 2-demultiplexing filter bank 82, and the third stage 2-demultiplexing filter banks 86 and 87 are connected to the second stage 2-demultiplexing filter bank 83.
As shown in FIG. 3A, each of the 2-demultiplexing filter banks is configured by a low-pass filter and a high-pass filter each performing band limitation for input signals, and down-samplers connected to an output of the low-pass filter and an output of the high-pass filter. The down-sampler has a function for alternately thinning out sample values of the input signal, that is, down-sampling a sampling frequency into half.
In the digital signal demultiplexing apparatus disclosed in the patent document 1, frequency characteristics of each low-pass filter of the 2-demultiplexing filter banks 81, 82, 84 and 86 correspond to a filter A shown in FIG. 4, and frequency characteristics of each high-pass filter of the same 2-demultiplexing filter banks correspond to a filter B in FIG. 4, and frequency characteristics of each low-pass filter of the 2-demultiplexing filter banks 83, 85 and 87 correspond to a filter C shown in FIG. 4, and frequency characteristics of each high-pass filter of the same 2-demultiplexing filter banks correspond to a filter D in FIG. 4. In addition, a 2-demultiplexing filter bank including the filter A and the filter B is connected as a subsequent stage of the filter A or C, and a 2-demultiplexing filter bank including the filter C and the filter D is connected to a subsequent stage of the filter B or D. By the way, the frequency characteristics of each filter shown in FIG. 4 are obtained by performing normalization using a sampling frequency fs of the signal input to the 2-demultiplexing filter bank.
Each of the 2-demultiplexing filter banks divides an input signal into two signals on the frequency axis using the low-pass filter and the high-pass filter, and down-samples each of the divided signals to output them. The digital signal frequency demultiplexing apparatus shown in FIG. 1 performs demultiplexing processes by hierarchically connecting a predetermined number of stages of these 2-demultiplexing filter banks. In the configuration, the predetermined number of the stages is log2N for a maximum number N for demultiplexing processes.
FIG. 5 is a diagram showing relationship between frequency bands and frequency positions for signals that can be processed by the digital signal demultiplexing apparatus for demultiplexing up to eight signals shown in FIG. 1. Signal names in the figure correspond to signal names shown in FIG. 1. By the way, a sampling frequency Fs in FIG. 5 is a sampling frequency of an input signal to the 2-demultiplexing filter bank 81 of the first stage.
FIG. 2 is a block diagram of a digital signal multiplexing apparatus described in the patent document 1. According to FIG. 2, in the digital signal multiplexing apparatus, 2-multiplexing filter banks 91-97 each having two inputs and one output are connected hierarchically. More particularly, 2-multiplexing filter banks 91 and 92 of the first stage are connected to a 2-multiplexing filter bank 95 of the second stage via selectors 98, 2-multiplexing filter banks 93 and 94 of the first stage are connected to a 2-multiplexing filter bank 96 of the second stage via selectors 98, and the 2-multiplexing filter banks 95 and 96 of the second stage are connected a 2-multiplexing filter bank 97 of the third stage via selectors 98.
As shown in FIG. 3B, each of the 2-multiplexing filter banks includes up-samplers, provided for each of the two input signals, for doubling the sampling frequency by interpolating a sample value of 0 between sample values of the input signal, a low-pass filter for limiting the band of an output signal from one up-sampler, a high-pass filter for limiting a band of an output signal of another up-sampler, and an adder for adding an output signal of the low-pass filter and an output signal of the high-pass filter to output the added signal.
In the digital signal multiplexing apparatus disclosed in the patent document 1, frequency characteristics of each low-pass filter of the 2-multiplexing filter banks 91, 93, 95 and 97 correspond to the filter A shown in FIG. 4, and frequency characteristics of each high-pass of the same filter banks correspond to the filter B in FIG. 4. Frequency characteristics of each low-pass filter of the 2-multiplexing filter banks 92, 94 and 96 correspond to the filter C shown in FIG. 4, and frequency characteristics of each high-pass filter of the same filter banks correspond to the filter D in FIG. 4. A 2-multiplexing filter bank including the filter A and the filter B is connected to a previous stage of the filter A or C, and a 2-multiplexing filter bank including the filter C and the filter D is connected to a previous stage of the filter B or D. By the way, frequency characteristics of each filter shown in FIG. 4 are obtained by performing normalization using a sampling frequency fs of an output signal of the 2-multiplexing filter bank.
Each of the 2-multiplexing filter banks up-samples each of the two input signals into doubled sampling frequency, and removes unnecessary harmonic component using the low-pass filter and the high-pass filter to frequency-multiplex and output signals such that they are arranged adjacent with each other on the frequency axis. The digital signal multiplexing apparatus shown in FIG. 2 performs multiplexing processing by connecting a predetermined number of stages of these 2-multiplexing filter banks hierarchically. The predetermined number of the stages is log2N for a maximum number N for multiplexing processes. In addition, the selector 98 connected to an output of each 2-multiplexing filter bank is for selecting a signal between a frequency multiplexed signal output by the 2-multiplexing signal and a signal having a bandwidth same as that of the frequency multiplexed signal, and an output of the selector 98 becomes an input signal of a 2-multiplexing filter bank of a next stage.
FIG. 5 is a diagram showing relationship between frequency bands and frequency positions for signals that can be processed by the digital signal multiplexing apparatus for multiplexing up to eight signals shown in FIG. 2. Signal names in the figure correspond to signal names shown in FIG. 2. By the way, a sampling frequency Fs in FIG. 5 is a sampling frequency of an output signal of the 2-multiplexing filter bank 97 of the last stage.
[Patent document 1] Japanese Patent No. 3299952